Bilirubin, a catabolic product of red cells and hemoproteins, is normally detoxified by conjugation with glucuronic acid in liver through the glucuronyl transferase system. It is an extremely toxic compound to some tissue cells and so it is bound with serum albumin to prevent its entry into tissues - other than the liver - to where it is transported via the vascular stream. However, the hepatic glucuronyl transferase system for the detoxification of bilirubin, as well as other parts of the degradative pathway is only slowly developed in neonatal life. Thus, if red cells are broken down at an abnormally high rate, the bilirubin accumulates and can rise to heights where it may exceed the albumin binding capacity and then enter body cells. The brain is especially vulnerable to the deleterious effects, some of which include inhibition of respiration, uncoupling of oxidative phosphorylation, abolition of respiratory control, large irreversible swelling of mitochondria, and rapid loss of cellular ATP and viability. This project is designed to determine the following: (a) the mechanism of the cellular toxicity of bilirubin, (b) the factors that govern the binding of bilirubin to albumin, and (c) how to apply the information obtained from (a) and (b) clinically to the hyperbilirubinemic newborn so that no newborn will suffer brain damage from this bile pigment. A variety of subcellular organelles, cells, tissues and whole animals will be used to measure the metabolic functions altered by bilirubin. Spectrophotometric optical rotatory dispersion and circular dichroism, enzymic and other biochemical and metabolic methods will be employed to study the nature of bilirubin and bilirubin-albumin bond under a number of altered situations such as pH, molar ratios, in presence or absence of lipid, ionic strength and other parameters simulating physiological and clinical conditions. Artificial membrane systems will be a tool to determine the effects of bilirubin on membranes. An artificial conjugated bilirubin will also be employed in all these systems to determine protein binding, toxicity or competition with unconjugated bilirubin.